A conventional plunger/pump will now be discussed with a reference to FIG. 8.
In FIG. 8, a reciprocating plunger 2 in a cylinder 1 sucks a liquid through a sucking inlet pipe 4 and discharges it from a discharge pipe 5. Numeral 2a is a connecting rod and numeral 3 is a crank shaft for converting its rotation movement to a reciprocal movement of the plunger 2. Numeral 1a is a sucking chamber, in which a certain amount of sucked liquid remains until it is discharged from the discharge pipe 5. Numeral 1b is slidable means disposed upon an inner wall of the cylinder 1 and upon an outer wall of the plunger 2. At the moment the plunger 2 descends, the liquid is supplied into the sucking chamber 1a through the sucking pipe 4, which at the moment the plunger 2 ascends, it is discharged from the sucking chamber 1a to the discharge pipe 5.
A decisive disadvantage of such a conventional plunger pump is that since the slidable means 1b on the inner wall of the cylinder 1 and on the outer wall of the plunger 2 is directly communicated with the sucking chamber 1a, the contaminants occurring from the sliding result of the outer wall of the plunger 2 and the inner wall of the cylinder 1 are mixed with the liquid to be highly pure.
In particular, the liquid which is used in the purifying process when manufacturing medicines, chemical products, semiconductors or the like must maintain a high purity, but the liquid flowing in the interior of the conventional plunger pump is contaminated by such contaminants i.e. worn matters.
For instance, in the purifying process of semiconductor products, it is known that their yield is greatly dependent upon maintaining a high purity level of the purifying liquid as an ultrapure water. For this purpose, it is one way of solution to filter the liquid which has been discharged from the cylinder 1. However, one problem is that after the liquid has passed through a filter device, its pressure is decreased, thereby its feeding efficiency is reduced. A further problem is that since the liquid is flowing in a high pressure, the filter medium in the filter device will be rapidly wasted.